Heartgard Plus for Dogs, up to 25 lbs, Blue, 6 Chewables Heartgard Plus for Dogs up to 25 lbs, 6 Month (Blue) contains ivermectin, which is a broad-spectrum antiparasitic that kills heartworm larvae in the tissue stage before they can cause potentially fatal heartworm disease. Heartgard Plus petmeds also contain pyrantel pamoate to eliminate and control hookworm and roundworm infestations.